Looks like it adds a slight benefit to bringing a fire and a frost mage to a raid, since one can stack winter's chill/whatever, and the other can put up fire vulnerability.
Either way, I think i'd try out arcane, some nice additions, plus it's been a while.
What? Frostfire from my understanding, just hits, calculates which would do more damage (the frost aspect of the bolt or the fire aspect of the bolt) then applies that damage. I doubt very much that it will gain both the +fire damage and +frost crit bonuses. More than likely it would benefit from one or the other.